Trihedral structures have found broad application in optical devices. In one typical application, these structures are arranged in a pattern to provide a retroreflector capable of reflecting light back along its incident path over a wide range of incident angles. These structures may be transparent, relying on internal reflection, or coated with reflective material. To achieve optimal results, it is often desired that the trihedral structure resemble a cube corner as closely as possiblexe2x80x94having three mutually perpendicular surfaces that are optically flat.
For some applications, cube corners are configured to retroreflect light in a designated pattern or divergence profile. U.S. Pat. Nos. 4,938,563 to Nelson et al. and 4,775,219 to Appeldorn et al. are cited as representative examples of this type of device. Retroreflectors have been used to produce flexible reflective tapes, road signs, and various safety devices. In addition, retroreflectors have been arranged to convey information. U.S. Pat. Nos. 4,491,923 to Look and 4,085,314 to Schultz et al. are cited as examples of this type of arrangement. Indeed, a wide variety of systems have been proposed which incorporate cube corner reflective elements such as the optical scanner of U.S. Pat. No. 5,371,608 to Muto et al. and the satellite defense system of U.S. Pat. No. 4,852,452 to Barry et al.
Frequently, retroreflective devices are mass produced from tooling patterned after the corner cube structure of a master mold. For instance, cube corner retroreflective sheeting is manufactured by first making a master mold that includes an image of a desired cube corner element geometry. This mold may be replicated using, for example, an electrochemical replication process such as nickel electroplating to produce tooling for forming cube corner retroreflective sheeting. U.S. Pat. No. 5,156,863 to Pricone, et al. provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Conventional methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and laminate techniques. Each of these techniques has various limitationsxe2x80x94especially when both small cube corner dimensions and high optical performance are desired.
For the direct machining approach, grooves typically are formed in a unitary substrate to form a cube corner retroreflective surface. U.S. Pat. Nos. 3,712,706 to Stamm and 4,588,258 to Hoopman provide illustrative examples of direct machining techniques. Direct machining techniques offer the ability to accurately machine very small cube corner elements (e.g. less than about 1.0 millimeters) which is desirable for producing a flexible retroreflective sheeting. However, it is not presently possible to produce certain cube corner geometries that have a very high effective apertures at low entrance angles using direct machining techniques. By way of example, the maximum theoretical percent active aperture of the cube corner element geometry depicted in U.S. Pat. No. 3,712,706 is approximately 67%. U.S. Pat. Nos. 5,600,484 to Benson et al., 5,585,164 to Smith et al., and 5,557,836 to Smith et al. are cited as additional examples of various cube corner machining techniques.
To overcome these limitations, the surfaces of each cube corner should be optically flat and should join adjacent surfaces at well-defined anglesxe2x80x94even if spacing between adjacent cube corners is on the order of a few hundred micrometers. Thus, there is a need for a more precise corner cube array. Preferably, the device may be provided as a unitary piece and is readily applicable to replication techniques. In addition, it is preferred that a technique be provided to form an array of cube corner-shaped microstructures having cube corner spacing of less than about 200 micrometers. The present invention meets these needs and provides other important benefits and advantages.
The present invention relates to corner cube structures. Various aspects of the invention are novel, non-obvious, and provide various advantages. While the actual nature of the invention covered herein may only be determined with reference to the claims appended hereto, certain features which are characteristic of the preferred embodiment disclosed herein are described briefly as follows.
In one feature of the present invention, a corner cube array is provided that includes a (111) silicon substrate and a number of silicon crystal projections generally extending from the substrate along a [111] crystal lattice direction. The projections each have a cube corner shape with three generally planar surfaces. The surfaces are generally mutually perpendicular and generally correspond to (100), (010), and (001) crystal faces. The projections each have generally the same size and shape and have a generally uniform distribution along at least a portion of the substrate.
In another feature, a silicon substrate has a generally cubic crystal lattice and a number of elements are generally positioned apart from one another in a predetermined spatial pattern along a plane of the substrate. These elements are made from a compound selected to spatially control silicon crystal growth on the substrate. A number of silicon crystal projections extend from the plane. These projections each have three generally planar surfaces. The projections are spaced apart from each other in accordance with the predetermined pattern of the elements to provide a corner cube array.
In still another feature, a crystalline substrate is selected having a generally planar first surface substantially corresponding to a first crystal face. A predetermined spatial pattern is defined along the first surface to control crystal growth thereon. A material is deposited on the first surface to grow a number of crystals corresponding to the pattern. The crystals have generally the same chemical composition and crystal lattice arrangement as at least a portion of the substrate. The crystals extend from the first surface to define second, third, and fourth generally planar surfaces. The second, third, and fourth surfaces substantially correspond to second, third, and fourth crystal faces. The second, third, and fourth crystal faces are oblique relative to the first crystal face. This technique may be utilized to provide a corner cube array structure useful to make replication tooling. The replication tooling may be operated to provide a number of articles each having a corner cube array shape.
In a further feature, a corner cube array is made by processing a silicon substrate having a cubic crystal lattice. A number of crystal growth regions are established along the surface during processing. These regions are established in a predetermined pattern. A cube corner shaped projection is epitaxially grown on each of the regions. The projection generally extends along an [111] crystal lattice direction with three generally planar surfaces. The surfaces are generally mutually perpendicular to one another and substantially correspond to (100), (010), and (001) crystal faces. This crystal growth technique may be utilized to provide a corner cube array with cube edges less than 200 micrometers in length.
Accordingly, it is one object of the present invention to provide a crystal corner cube array.
It is another object of the present invention to grow a cube corner having crystal faces that are oblique relative to a crystal face of a substrate on which the cube corner is grown.
It is still another object of the present invention to provide cube corners spaced apart from each other by distances of less than about 200 micrometers.
It is yet another object to provide a crystal corner cube array suitable for making replication tooling.
In an additional object, a corner cube array is grown on a planar surface of a substrate using Selective Epitaxial Growth (SEG) and Epitaxial Lateral Overgrowth (ELO) techniques.